Room: Exhibit Hall
Purpose: To optimize the selection of the cone size and the isodose line(IDL) in order to achieve high quality linear accelerator based stereotactic radiosurgery (SRS) plan.
Methods: Two artificial spherical PTVs, with diameter of 10mm and 20mm, were created on CT images of a head phantom. Five arc fields were used for SRS planning. Corresponding to field margins of 0mm and 2mm, 10mm and 12mm cone sizes are chosen for 10mm PTV, while 20mm and 22mm cone sizes are chosen for 20mm PTV. For each PTV, doses were normalized to achieve the same target conformality index(CI)with the two cone sizes. Dosimetric comparisons among plans were performed with parameters of prescribing isodose line, PTV dose uniformity, dose gradient, the maximum dose to the brain and the total monitor units to deliver plans.
Results: Corresponding to the field margins of 0mm and 2mm, the IDL chosen for dose prescription increases from 70% to 87.7% for 10mm PTV, 75% to 87.2% for 20mm PTV. The minimum doses increase accordingly from 71.3% to 88.8% for 10mm PTV, 73% to 85.8% for 20mm PTV. The mean doses increase from 90.6% to 95.8% for 10mm PTV, 92.8% to 95.4% for 20mm PTV. Dose gradients increase from 1.9mm to 2.4mm for 10mm PTV, 3.1mm to 3.5mm for 20mm PTV. The maximum dose to the brain decrease from 129% to 110.2% for 10mm PTV, 122.3% to 110.9 for 20mm PTV. The total monitors to deliver 20Gy dose to target decrease from 6406 to 3906 for 10mm PTV and from 4944 to 3613 for 20mm PTV.
Conclusion: In SRS treatment planning, clinician could try different field margins according to the PTV size and location of surrounding critical organs to optimize the dose delivered to the patient.